Temperature acclimation of growth, photosynthesis and respiration in two mesophilic phytoplankton species

Temperature acclimation in two mesophilic microalgae, Microcystis aeruginosa (Cyanobacteriales) and Scenedesmus acutus (Chlorococcales), was studied by measuring growth rate, photosynthesis, respiration, cell size, cellular pigment content and Chl a-specific light absorption. Phytoplankton were grown as nutrient-replete semicontinuous cultures for 2 weeks at 5, 15 and 25 degrees C, during which growth rate was determined from changes in Chi a. Gross photosynthesis (GP) was measured as C-14 assimilation at saturating light and respiration (R) was measured as O-2 uptake along a temperature gradient from 0 to 40 degrees C. Net photosynthesis (NP) was determined as the difference between GP and R. For both species, acclimation to increasing growth temperatures resulted in increasing growth rate, cellular pigment content and decreasing cell size and Chi a-specific light absorption. Scenedesmus acutus and M. aeruginosa showed the same overall pattern of metabolic acclimation to increasing temperatures: (1) overall higher GP and NP but lower R; (2) increasing optimum temperatures for GP, NP and R and (3) higher metabolic rates at supraoptimal temperatures. Microcystis aeruginosa showed several warm-loving traits. It was more sensitive to increasing temperatures (higher Q(10) values), had higher metabolic rates and optimum temperatures and performed better at high incubation temperatures than S. acutus did. This study shows that phytoplankton have a considerable and rapid ability to adjust cellular physiology, metabolism and growth to relatively large changes in growth temperature. This suggests a significant ability to acclimate to increasing temperatures associated with forthcoming climate changes.